Project I focuses on the health and development of Mexican children of immigrants ages 0 to 5. Mexican origin children are known to be at disproportionate risk for a number of physical, behavioral, and developmental disorders, and research addressing these health issues has been described as an urgent priority. The broad aims of Project I are to investigate how immigration and assimilation processes shape the family circumstances of Mexican-origin children, and to analyze how family circumstances, in turn, shape children's health outcomes. The project will use data from the birth cohort ofthe Early Childhood Longitudinal Study (ECLS-B), the National Health Interview Survey linked to Office of Immigration Statistics records (NHIS-OIS), and the Mexican Family Life Survey (MxFLS) to: (1) determine the relative merits of various approaches to the conceptualization and measurement of young children's health, with an emphasis on understanding interethnic health disparities; (2) demonstrate the role of family contexts in interethnic health disparities; and (3) investigate how immigration influences family contexts, and how immigration and family contexts jointly shape intraethnic health disparities. The project will classify children across health conditions as well as within conditions; unpack race-ethnicity and immigration experience in terms of specific aspects ofthe family context that may affect young children's health and development; follow individual children longitudinally to assess the links between immigration, assimilation, the family context, and child health; investigate the role of parents' legal immigration status; and place the study of U.S. Mexican-origin children in a broader context using binational data. The project is highly relevant to public health because of the rising number of Mexican-origin children of immigrants and limited knowledge about their health and development.